In high availability computing architectures, a widespread way of protecting important data from damage or loss, in case of a disaster, is represented by the mirroring technique. Said technique provides for maintaining at least two copies of important information stored in at least two different storage units arranged in two different sites: a first local copy, which generally represents the “work copy”, is directly used by a software application running in a main computing system (computer) located in the Primary Data Centre (PDC), while a second copy is located in the remote Recovery Data Centre (RDC) and is used within the ambit of a disaster recovery process in case of failure of the main computing system.
There are known in the art at least two methods for carrying out data set replication: a first technique or method, known as synchronous mirroring, wherein the updating of a remote copy at the RDC is concurrent with the modification of a data item in a local copy at the PDC;
a second technique, known as asynchronous mirroring, the updating of the remote copy occurs in accordance with a batch policy.
The synchronous mirroring technique, which is the relevant art for present invention, provides, in general, for the following steps:    a—writing a data item on a local storage unit;    b—writing the data item on a remote storage unit;    c—waiting for an acknowledgement signal ACK from the remote disk before repeating steps a) and b) by writing again a new data item.
The synchronous mirroring gives more guarantee of recovering the status of the main computing system in case of failure, data loss or disaster, because it allows to have at least two storage units perfectly aligned in every moment.
In case of a disaster, by using synchronous mirroring it is possible to keep low the so called Recovery Time Objective or RTO, that is the time interval necessary for restoring the normal working of a software application identical to the applications running at the PDC before the disaster.
In fact, by adopting a synchronous mirroring technique, it is granted to reduce the deviation, between a main data set and its copy useful for restarting the software application, ideally at one transaction.
In the art, the deviation, measured in time, between the main data set and its copy, is usually named Recovery Point Objective or RPO.
It is to point out that the sequence of steps relative to the synchronous mirroring requires that the software application at PDC is locked for a time interval elapsing from the production of a data item by the software application itself to the receipt at PDC of an acknowledgement of the writing of the data item (writing operation) by the remote storage unit at the RDC. This time interval can be estimated as the sum of:                the serialisation time of the data item;        the round trip time, that is the sum of the propagation delay and of the processing and queuing time in the apparatuses existing along the connection between the main computing system and the remote copy;        the writing time of the data item on the disk into the RDC;        the production and serialisation time of the acknowledgement signal, said time being negligible with respect to the serialisation time of the data item and the round trip time.        
In general, the minimum round trip time can not be lower than the time interval relative to the physical propagation (propagation delay) in the used medium, said time interval being directly proportional to the distance on the physical connection between the computer in the Primary Data Centre (PDC) and the remote storage unit in the Recovery Data Centre (RDC).
It is well known within the storage industry and thoroughly documented in publications by storage software and hardware vendors that synchronous mirroring slows down the performance of the software applications producing the data involved in synchronous replica (mirroring). As the distance between PDC and RDC grows, applications performances decrease in a proportional way. As an example, assuming that the only limiting effect is the propagation delay, an increase of the distance between PDC and RDC from 10 km to 100 km provides an increase of the write response time (round trip time) by a factor of 10; as a consequence the application throughput can be reduced, depending on the amount of the rate of the write operations, up to 90%.
Anyway, Applicant believes that the distance limit beyond which those performances become unacceptable to sustain the normal functionality of the computing system at PDC cannot be defined in a simple and unique way, because it strictly depends on the kind of business and related software applications (size and frequency of the write operations) and on physical characteristics of the communication network between PDC and RDC (bandwidth, technology and topology).
Some literature sources indicate some distance limits about synchronous mirroring:                Nortel Networks white paper “Storage Distance extension: increasing the reach and utility of networked storage applications”, indicates 400 km as the distance limit even when using high-bandwidth links;        Hitachi white paper “Business Continuity Solution Blueprint—Synchronous data replication” indicates 50 km, states that the distance limit depends in particular on the application response time;        Proprietary solutions for synchronous mirroring indicate different distance limits due to the particular software managing the data replication; more in particular IBM PPRC (White Paper “IBM and Cisco: Metro Optical Solution for Business Continuity and Storage Networking”, August 2003) indicates 40-100 km as distance limit. EMC SRDF (White Paper “EMC and Cisco Metro Optical Storage Networking Solution”, 27 Jun., 2001) indicates 80 km as the maximum distance limit.        
Applicant notes that, even in presence of high bandwidths, whereby the serialisation time is negligible, and in presence of dedicated circuit connections, whereby the round trip time is reduced at a minimum, the synchronous mirroring technique can not in general be applied to connections having any distance between PDC and RDC.
Such a limit contrasts with the typical requirement of high availability computing architectures according to which it is necessary to store the replication of the data in a site located at large distance, e.g. some hundreds of kilometres from the work copy, so as to allow an higher level of protection in case of huge disaster.
In order to getting round the above problem intrinsic to the synchronous mirroring technique, a technique known as multi-hop disk mirroring has been proposed. Such technique has been described for instance in “Asynchronous Cascading Implementations, TIPS0310”, IBM Redbook—Hints & Tips, Oct. 15, 2003, available in Internet on Jun. 14, 2004 at the Web site http://publib-b.boulder.ibm.com/Redbooks.nsf and in “Remote Mirroring of Business Critical Information”, EMC, Jun. 10, 2002, available in Internet on Jun. 14, 2004 at the Web site http://italy.emc.com/local/it/IT/download/pdf/giugno2002/Burns.pdf.
The multi-hop mirroring technique provides for carrying out a synchronous mirroring at an intermediate site located at a distance compatible with the limits imposed by the software application at the PDC and for replicating the data asynchronously towards the RDC site.
Applicant notes that the multi-hop mirroring has the drawback of introducing complex elements in the end-to-end chain.
According to the known art, multi-hop mirroring requires to introduce storage units in the intermediate site, which must have at least the same storage capability of the storage unit in the PDC.
Such a solution reduces the overall reliability of the architecture.
Moreover, since the updating from the intermediate site to the recovery site (RDC) typically occurs in batch mode at a relatively low frequency, relevant differences can arise between the main data sets and the recovered data sets.
In case of disasters having a range of action including both the primary site (PDC) and the intermediate site, it could be, therefore, impossible of obtaining a very low Recovery Point Objective or RPO.
The paper “Heterogeneous Midrange Storage with Local Mirroring and Remote IP Replication”, Falconstore, Dec. 10, 2002, available in Internet on Jun. 14, 2004 at the Web site http://www.falconstor.co/Whitepapers/ MidrangeSSFSolutionWhitePaper.pdf and the PCT patent application no. WO 02/069159 discloses a mirroring technique wherein it is provided a device located at PDC and inserted between the main computing system, running the software application, and the local storage unit. The copy on the remote storage unit always occurs asynchronously through the device located at PDC.
In summary, Applicant believes that no solution is known in prior art for carrying out a synchronous mirroring independent from the distance between PDC and RDC and having a very low RPO.